Fix typo in assigning the default IMPALA_BUILD_THREADS
Fix typo in assigning the default IMPALA_BUILD_THREADS

A '$' was missing so IMPALA_BUILD_THREADS would not be set. Also change
to use the ':=' version since IMPALA_BUILD_THREADS is defined but null
in people's environments (due to the export in the line below).

Change-Id: I0966a409f61ab5d54c09b71e9ed149d561fa43ae
Reviewed-on: http://gerrit.cloudera.org:8080/2454
Reviewed-by: Alex Behm <fe1626037acfc2dc542d2aa723a6d14f2464a20c@cloudera.com>
Tested-by: Internal Jenkins

kapilrastogi/Impala,kapilrastogi/Impala,kapilrastogi/Impala,kapilrastogi/Impala,kapilrastogi/Impala,kapilrastogi/Impala,kapilrastogi/Impala